Mr McGowan accused the government of "chickening out" of its responsibility to keep pace with population growth, with Perth set to double in size over the next 30 to 40 years.

"They're spending an enormous amount of taxpayers' money and they're not doing it right the first time around," he said.

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Last month, a Senate estimates committee heard that there was an effective deadline of November 8, after which construction would be too advanced to allow the design to be changed.

Flanked by dozens of protesters brandishing signs describing November 8 as "D-day", Mr McGowan said Dr Hames should make the decision now, rather than adding a floor when the hospital was already operational.

"It's a ludicrous, stupid, ridiculous proposition that you're somehow going to take the roof off a hospital with gravely ill children in it, some of whom are having surgery where any vibration could potentially cause them grave injury, and then somehow rebuild the hospital that way," Mr McGowan said.

"This is a government that's doing it on the cheap. The fact of the matter is, this government lost the AAA credit rating, and now children and families are suffering."

A spokesperson for Dr Hames said the latest version of the plan will involve the future installation of between one and four new partial floors above the building's central column.

"Disruption to normal hospital operations as a result of expansion would be manageable as this area is not above in-patient areas," the spokesperson said.

Mr Parish said he had been advised by construction contractor John Holland that adding an extra floor would be possible now, but not once the building had been completed.

If extra capacity was not added to the new hospital, Mr Parish said he would begin campaigning for another new one.

"In two years time when the new hospital is full, we'll be campaigning for another children's hospital - how crazy is that?" he said.

Mr Parish, who lost his son Elliot to cancer in 2011, said the existing cancer ward at Princess Margaret Hospital was 20 per cent oversubscribed. But the new hospital was set to have only two more beds.

"You're going have a ward that's overcrowded from the day the doors open, and that extends to all the wards in the hospital," he said.

Dr Hames' office has denied that facilities at PMH were overstretched, pointing out that the average occupancy over the past year was 78 per cent.

Overall, the new hospital will contain more than 50 per cent more clinical and research area than PMH, and its emergency department will be 88 per cent bigger. Dr Hames said modelling suggested the health system would be able to keep up with patient demand until 2021.

Dr Hames also said the new hospital would be the centrepiece of a "hub and spoke" model that also included six satellite hospitals with dedicated paediatric beds throughout the metropolitan area.

But Mr McGowan and Mr Parish both claimed that parents with children who need emergency care would prefer to take their children to the dedicated children's hospital, rather than their nearest facility.